Internal turning operations differ significantly from external turning operations in several aspects which must be taken into consideration if internal turning operations are to be optimized. Firstly, there is a limited space inside the workpiece. This influences the design of the cutting tool; it is essential that the optimum compromise between maximum strength/rigidity and minimum volume be found. In practice, one solution involves the provision of a bar, which is cylindrical in cross section, and in practice most bars are substantially cylindrical although one or more longitudinal planar surfaces which are parallel to the longitudinal axis of the bar may be incorporated. Said planar surfaces are used to fix the rotational position of the bar in the tool-clamping device. Thus the need to be able to fix exactly the rotational position of the bar and hence the height of the operative cutting edge in such cases is achieved at the expense of a somewhat weaker design of the bar, since by machining the planar surfaces into the bar, the amount of material in the bar is reduced.
The limited space inside the workplace within which the tool must operate also makes chip forming, chip flow and chip breaking far more critical than in external turning operations since the failure of any of these chip-related activities can lead to chip jamming, damaged tools, poor finish on the machined surfaces and tool vibration. Tool manufacturers put considerable effort into designing cutting inserts and tool holders for internal turning operations which minimize those problems, but to be successful it is essential that the position of the cutting edge relative to the workplace be exactly as the manufacturer intended.
Secondly, internal turning operations differ from external ones in that vibration of the tool is always present and it has a major influence on tool life, surface finish and productivity. Incorrect positioning of the cutting edge may lead directly to cutting forces which differ from those for which the tool was designed, and the negative effects on chip forming will also often lead to vibration.
Swedish Patent 500 836 teaches the use of a spirit level in contact with a flat planar upper surface of a boring bar which is used to ensure that the operative cutting edge of the insert mounted on the cutting tool is clamped in a pre-determined position. This solution has the disadvantage (already mentioned) that the machining of the planar surface decreases the material in the bar and hence its strength. Furthermore the use of a spirit level is not desirable because of the limited space and poor accessibility in a modern machine tool plus the difficulties of reading a spirit level in the presence of cooling fluid and chips.
International Publication WO 95/35179 teaches the use of a partially cylindrical boring bar with two planar longitudinal surfaces running parallel to the longitudinal axis of the bar. Said surfaces abut against matching surfaces in a “V” shaped groove in the tool block. This solution also involves a weakening of the bar as described in the previous example.
Swedish Patent 509 421 teaches the use of a setting device to determine the angular position of a cutting tool and cutting edge. The cutting tool is locked in position in the device using a fixing element, which interacts with a cylindrical aperture in the envelope surface of the cutting tool. A rotatable disc with angular markings is then used together with a spirit level to determine the angular position of the cutting edge. This solution is time consuming to use and is not suitable for the limited space and poor accessibility in a machine tool.